1. Field of the Invention
The present invention relates to a transmission rate switching control method for a mobile communication system which permits a plurality of mobile stations to share the same resources and, during the communication process, switches a transmission rate for each mobile station in accordance with the amount of information transmitted and received. The present invention further relates to the mobile communication system and a mobile communication control apparatus.
2. Description of the Related Art
There is a well known transmission rate switching control technique for a “FOMA (product name)” system which employs CDMA (Code Division Multiple Access), a third generation mobile communication system.
This conventional mobile communication system provides a packet communication service at a maximum transmission rate of 384 kbps. However, since many resources are consumed to obtain the maximum transmission rate, an upper limit is imposed on the number of mobile stations which can simultaneously perform communication at the 384 kbps transmission rate. The resources in this case are a transmission power resource unique to the CDMA system and a scramble code resource. Many of both types of resources must be allocated to attain a high transmission rate. Accordingly, an upper limit is imposed on the number of mobile stations which can simultaneously perform communication at the 384 kbps transmission rate.
Therefore, to efficiently apportion the limited number of available resources among the mobile stations, the conventional mobile communication system employs a method in which for high speed communication requiring many resources, the transmission rate is reduced in accordance with the remaining number of resources so as to allocate the resources. This method prevents a phenomenon which occurs wherein the resources are exclusively used by a small number of mobile stations having high transmission rates, and call loss probability is increased.
Further, the conventional mobile communication system switches the transmission rate in accordance with the amount of information transmitted and received. That is, the conventional mobile communication system increases the transmission rate when a predetermined number of signals or more is stored in a transmission buffer, or reduces the transmission rate when a predetermined amount of information or less is transmitted. In this manner, only the required resources are controlled to be allocated at all times, and an efficient use of the resources is ensured.
However, since the conventional mobile communication system performs transmission rate switching control which alters the transmission rate in accordance with a change in the amount of information being transmitted or received, a switching control load imposed on the mobile station and the control station may be increased when the amount of information greatly fluctuates.
Furthermore, when a request for a switch to a high transmission rate is received in the circumstances where only a few resources remain, allocation of the resources is performed while the transmission rate is reduced. Accordingly, another request for a switch to a higher transmission rate may be received sequentially, and the control load may be increased.
As a countermeasure for these problems, a method is conceivable in which after switching a transmission rate, the following switching control is uniformly halted for a predetermined period of time. However, when this technique is employed, the occurrence of other problems is predicted, that is, throughput may be reduced due to a delay in switching to a high transmission rate, and since a processing time for switching from the high transmission rate to the lower transmission rate is extended, resource use efficiency may be deteriorated, so that the resources can not be allocated to another mobile station requiring a high transmission rate.